There are a large number of applications that require the integration of environmental sensors with data processing capabilities. For example, for some applications the external pressure should be detected and then the pressure information transmitted to another device. In fact, as the Internet of Things (IoT) becomes more pervasive, many of the networked smart devices will be, and already are, sensors that capture and measure their surrounding environmental properties and then report that information to a server on a cloud system, for example.
A fundamental contradiction arises, however, when integrating environmental sensors with other data processing capabilities. Semiconductor dies are often packaged to protect the dies from the environment and to prevent damage. On the other hand, sensors must typically be directly exposed to the environment to detect pressure or sense/analyze gases or other fluids, for example.
Nevertheless, a number of techniques have been developed that allow for the integration of environmental sensors with other integrated circuits and systems. U.S. Pat. No. 9,176,089 B2, to Le Neel at al., “Integrated Multi Sensor Module”, 2015, describes a multiple environmental sensor module. Also, U.S. Pat. No. 7,981,698 B2, to Pryputniewicz et al., “Removing Integrated Circuits from Packaging”, 2011, involves removing the plastic packaging and removing the encapsulating dielectric.
At the same time, the use of redistribution layers (RDL) is somewhat common when flip chip mounting and solder bumps are used. RDL is primarily used when it is required to move the location of bond pads for a given integrated circuit die. Sometimes, when the integrated circuit is designed, the location of the bond pads is fixed. Then later, when that die must be mounted in a package, the bond pads should be moved to a different location. This can happen for a number of reasons. It can be important to spread the contact points around the die so that the stress from the solder balls can be appropriately distributed. RDL is also used in die stacking. In this application similar dies are mounted in a common package. Each die must be wired differently, however, because of possibly addressing requirements. In this case, different address lines can be connected differently for each of the respective dies in the common package. Additionally, RDL can also be used to move the bond pads to more convenient locations. It can even be used to provide compatibility between different dies when the location of the original bond pads makes them incompatible
In the typical RDL process, a dielectric layer such as polyamide is deposited over the integrated circuit die. This layer is then patterned to open access to the die's typically aluminum or copper bond pads. This polyimide layer may be several micrometers thick. Conductive metal layers are then deposited between the original bond pad locations and the desired locations for the bond pads/solder balls. Then, a second polyamide layer is deposited over the metal layer, covering the old bond pad and providing an opening at the new bond pad location. The solder ball or bump or the wire bond can then be attached to the new bond pad location.